Salt annual world production of 250 million tons is used primarily by the chemical industry (70%), the food industry (10%), road deicing (10%) and (10%) for other uses. The Salt Institute has listed the ways salt is produced as including underground mining of salt deposits, solar evaporation of salt containing water from seas and lakes. In another method known as solution mining, brine is produced underground by pumping water to dissolve salt deposits and then pumping the dissolved brine to the surface or mechanical evaporation where salt is concentrated by any of a number of methods including heating, vacuum, precipitation. Another source of brine is that produced as a byproduct of desalination water treatment plants where the need is to efficiently vaporize the desalination liquid and where the salt may be of no value (it may be too contaminated from the chemical additives in the water treatment process). In this application is very important for inland desalination plants to achieve zero liquid discharge as an inexpensive environmentally friendly alternative. Without the ability to vaporize the brine, it would otherwise need to be spread over a large area with an expensive sealed bottom barrier from which a liquid can evaporate so as to not seep into a groundwater aquifer. Other process steps for all of the above recovery methods may include removal of impurities with recycling of heat for higher efficiency.
In all of the above production methods energy is needed to heat, vacuum, pump, mix, and transport the product, and extensive coastal land is required. Removal of impurities requires either extensive foot print for solar Salinas, which rely on precipitation of undesirable salt constituents and zoological/biological process to reduce organic contaminations. Chemical processing under controlled conditions is required to precipitate and remove undesirable components. These processes require extensive investments, space, and energy and, save properly designed Salinas, tend to be polluting. Coastal areas now being used to evaporate sea water in salt ponds or Salinas are being encroached upon by tourism and urban development making the land too expensive for salt production.
The high cost of energy could be lowered by using renewable energy salt production, yet it is still expensive and requires unduly large land areas. Production methods that can achieve high product quality with maximum renewable energy and a smaller footprint, will have a more competitive edge and are more environmentally acceptable.
Where a pond or other holding body is used the sealing of the bottom of the pond is expensive, and the harvesting operation always produces some form of wear and tear on the pond bottom, ranging from gravity effects of equipment wear to inadvertent loss of integrity of sealing of the pond bottom. Filling the pond with brine and waiting for sun and wind to remove the water is a process requiring significant time which allows undesirable organic matter to grow and requires costly cleanup.
Production of salt by spraying through enhanced heat exchange has been described in U.S. Pat. No. 6,027,607 which is dependent on the availability and proximity of heat from an industrial plant, a significant limitation. This process may be more properly characterized as a method for spending waste heat than an economic process for the production of salt. Production of salt in a pond, as described by the patent, will also require dredging of the salt deposited in the bottom of the pond, an expensive process. Dredging can also produce colloidal clay particles ingrained into the salt crystals which requires expensive treatment.
Other processes, such as the Grainer process described in U.S. Pat. No. 2,660,236, rely on evaporation of water to concentrate and deposit the salt but the expense of fuel and complexity of exchanging heat and maintenance of piping and pumps end up with an expensive salt and potential failure due to mechanical maintenance problems.
In another process, U.S. Pat. No. 4,334,886 describes concentration of salt based on natural evaporation through recycling of the brine to cascade down a tower and adding salt to assist in crystallization. Even though the process uses natural heat source, it requires erection of a tower and sacrificing part of the salt to concentrate the brine which makes the process inefficient and difficult to automate for collecting.
One of the reasons that so many evaporation implements have tended toward enclosure is the difficulty of controlling brine spreading and evaporation systems. Brine is heavy and tends to have increased viscosity and is difficult to atomize. The use of a large footprint system tends to cause pooling, due to uneven distribution, at the source of brine feed.
It is therefore desirable to produce crystalline salt on the surface from efficiently evaporating brine by heat from the sun and wind on soils that may be porus without sealing the underlying surface and building retaining walls to contain the brine while it evaporates. It is also desirable that the crystallization of brine into salt is fast such that biological products are not allowed to form. If undesirable biological products form, they must be removed through expensive, time consuming and extensive land environmentally sensitive processes.